Recently, the automotive industry has developed solutions to protect pedestrians from serious injuries in the event of an impact with the front side of a vehicle. For example, it has been proposed to provide one or more airbags on the engine hood and/or on the windshield for deployment in the event of an impact with a pedestrian. Another safety measure for an impact with a pedestrian is for the engine hood to be inclined at an angle to catch the pedestrian.
Proper deployment of such safety measures obviously depends upon a reliable detection of the impact with a pedestrian, which also implies an unambiguous differentiation from impacts with other objects. Collision detection may thus involve different detection steps. First, a collision event has to be identified and second, it must be determined whether a pedestrian has been struck. There are several approaches for assessing a collision situation. By using pressure sensors arranged at suitable locations on the outer periphery of the vehicle, one may determine the impact location as well as the magnitude of the pressure load, which allows deciding whether measures to protect a pedestrian shall be initiated.
A collision detection system for pedestrian protection is for example described in U.S. Pat. No. 6,784,792.
It will be understood that an essential requirement for an efficient detection of impact is the use of reliable pressure sensors. The force that is to be detected by the sensor due to the impact with a pedestrian generally has a rather high magnitude and is to be measured within a very short time period. Furthermore, pressure sensors for use in pedestrian protection systems should ensure reliable operation during the vehicle service lifetime. They have to be robust with regard to operating conditions in a vehicle, e.g. they should be insensitive to electromagnetic interference. Moreover, they should be designed in such a way that their integrity can be easily verified by check routines.
Among the various types of pressure sensors, Force Sensitive Resistor (FSR) based sensors are largely used in the automotive industry for their robustness and ease of manufacture. A well-known type of FSR switching element comprises two carrier foils that are arranged at a certain distance from each other by means of a spacer having an opening defining an active area. A planar electrode is arranged in the active area on one of the carrier foils. On the other carrier foil, facing the electrode, there is a layer of semi-conductive material. When no pressure is applied on the switching element, the electrode and semiconductor material layer are maintained apart due to the spacer. If a sufficient activation pressure is applied on the sensor, the planar electrode is brought into contact with the semiconductor material layer. The contact resistance diminishes as the pressure increases.
Although such a technology has proven efficient in many applications, it is not optimal for sensing the whole pressure evolution of impacts, in particular with pedestrians. Indeed, the measuring range of such FSR-switches is too narrow and its dynamic not sufficient to detect a complete pressure pulse of high magnitude that is caused by collision with a pedestrian.